Composite membrane systems are known in the art of separations of components from fluids such as by reverse osmosis. Representative known art is U.S. Pat. No. 4,277,344 to Cadotte which teaches in situ preparation of polyamide films on porous supports useful for separation of salts from water by reverse osmosis. The polyamide film is formed at the interface of an aqueous solution of an amine, generally on a porous support and a polyacylhalide in a non-polar solution substantially incompatible with the aqueous solution of amine. The polyamide formation occurs substantially only at the interface.
Separation of gaseous components is also known by use of composite membranes. Representative gas separations composite membranes include U.S. Pat. No. 4,717,395 to Chiao which teaches membranes useful for the separation of gas components, including composite membranes having thin discriminating layer on a porous support. The discriminating layer is taught as separately formed and adhered on the support layer on a solution or dispersion of the discriminating material is coated or cast on a supporting layer and the solvent is removed to form a thin dense skin.
U.S. Pat. No. 4,230,463 to Henis et al. teaches a multicomponent membrane for separation of gas mixtures comprising a porous membrane coated with an amorphous material which wets or tends to adhere to the surface of the porous support and which demonstrates a selective permeability for a particular component of the gas mixture.
The prior art methods suffer from various handicaps. Coating of a support with a discriminating layer may result in variations of thickness of coating material. In areas where no coating is present the membrane is porous and therefore non-separating. Where the coating exceeds optimum thickness, an undue decrease of flux results. Where the discriminating layer is prepared separately, the discriminating layer frequently separates from the support, particularly under conditions where backflow occurs through the membrane, or there is turbulent flow near the discriminating layer.
A theoretical explanation of mass transport through a composite membrane for gas phase separation was developed by Lai, J-Y, et al., Journal of Polymer Science. Vol. 32, pp. 4625-4637 (1988). The rate of gas permeation through a composite membrane is stated as inversely proportional to the thickness of the composite layer. Thus a thin composite layer is desired. The instant invention provides a thin and substantially uniform selective polymeric layer on a porous support useful for separation of fluid mixtures.
Electrochemically initiated polymerization has been applied to deposit polymers upon electrodes for several end uses such as: corrosion protection of metals, preparation of display devices, immobilization of particulate catalysts, or preparation of ion specific sensors. In each case the application of electroinitiated polymer has been onto an impervious support. To date, however, no application has been reported wherein electroinitiated polymers have been applied to porous substrates.
Further, no report has been made of electroinitiated polymers being used as discriminating layers on porous supports for membrane separations.
The composite membranes of the instant invention are comprised of an electrochemically initiated polymerization of a discriminating layer on a porous support.